1. Field of the Invention
The present invention relates to an apparatus employing a driving mechanism provided with an electro-mechanical transducer such as an electrostrictive element, a piezoelectric element and a magnetostrictor for transducing an electric energy into mechanical displacement.
2. Description of the Prior Art
As a drive mechanism for moving a lens of a camera or the like into an in-focus position, there has been proposed a drive mechanism in which a driven member is put in frictional coupling with a driving axis fixed to a piezoelectric element with an appropriate frictional force, and the driving axis is reciprocally displaced by generating extension/contraction displacement at a varied speed in the piezoelectric element so as to move the driven member put in frictional coupling with the driving axis in a specified direction (refer to Japanese Laid-Open Patent Application No. Hei 4-69070). This drive mechanism has not only the feature of a remarkably compact size, light weight and low manufacturing cost as compared with those of the conventionally used drive mechanism comprised of a motor and a speed reduction mechanism but also the feature that it can execute a high-accuracy position control.
Furthermore, the specification of the U.S. Pat. No. 4,874,979 proposes one in which its driven member is moved by combining three piezoelectric elements with one another and appropriately controlling the timing at which extension/contraction displacement occurs in each of the piezoelectric elements.
In regard to the aforementioned structures of the drive apparatuses, the former is constructed so that the driving axis and the driven member are put in frictional coupling with each other, and when the driving axis is driven by the piezoelectric element, slip is generated between the frictional coupling surfaces of both of them and the slip is utilized for moving the driven member in a specified direction. However, since the amount of slip is not always constant, the position of the driven member cannot be controlled through an open loop, and it is required to execute feedback control with a means for detecting the position of the driven member. In this case, the position control cannot be executed finer than the resolution of the position detecting means. Therefore, in order to execute high-accuracy position control, a position detecting means having a high resolution appropriate for the required accuracy is necessary, and this results in increasing cost.
Furthermore, in the latter, the position of the driven member can be controlled through an open loop when the extension/contraction displacement amount of each piezoelectric element relative to an applied voltage is previously known. However, in addition to the disadvantage that the driving speed is slow, the number of components increases due to the use of three piezoelectric elements, and this results in increasing manufacturing cost.